A. Technical Field
The present invention relates generally to coupling data from a high-voltage line side to a low-voltage data transmission side of a system where the coupling is facilitated by means of a mutual inductance (for example a transformer), or by means of a set of optical couplers. Such systems have particular relevance to electricity usage meter applications.
B. Background of the Invention
The industry requirements for electricity usage metering are evolving rapidly and becoming more challenging for both the meter design companies and the providers of electricity metering equipment. The electric utility and the energy consumer want accurate metering, but also want cost effective solutions. With huge amounts of power measured constantly, each KWH that is monitored inaccurately represents significant money to the power companies or to the consumers.
Current schemes for collecting information from geographically distributed sources, for example utility meters, present a number of problems. Manual data collection methods such as those employing human meter readers are error prone and costly. Accordingly, utility companies have embraced automated data collection technologies via data networks. Wired data network interfaces, however, must be isolated from the line power line. This requirement demands a cost effective and electrically efficient method of coupling the metering information between the power line and the data network.
One of the challenges to effective implementation of a data network interface is that the metering environment is often noisy and subject to high-voltage spikes. These voltage spikes can be as high as 2K volts. The common solution is to provide the necessary isolation between the line and the data network by such devices as transformers or optical couplers. Both solutions have significant disadvantages.
In the case of line-side transformers, these devices are capable of isolating the voltages of interest and are typically wound on a split bobbin. Thus, transformers are large (occupying a great deal of board space), heavy and expensive. Also, transformers may introduce phase shift that must be compensated by other means in the meter. Further, transformers that connect directly to the line are likely to dissipate heat, which shortens the life of the equipment and constitutes a safety risk as insulating materials age.
Relative to optical couplers, if optical couplers are used in the analog data path, even the best optical couplers are highly non-linear, requiring hardware or software to linearize the transfer function. Moreover, If optical couplers are used in the digital path, the optical coupler must be fast enough to support the data rate of interest, typically greater than 10,000 bits, per second. However, inexpensive optical couplers can support only about half this data rate.
Accordingly, what is needed is a method to couple data from the high voltage line side to the low voltage data transmission side of a system, such as an electricity usage meter, that is accurate and cost effective.